he bottle in the vertical position (Fig. 204).

The apparatus being arranged is lowered to the required depth, a sharp
jerk is then given to the suspending cord, which detaches the rubber
tube and so opens the two glass tubes. Water enters through the longer
tube and the air is expelled through the shorter tube. The bubbles of
air can be seen or heard rising through the water, until the bottle is
nearly full, a small volume of compressed air remaining in the neck of
the bottle.

As the apparatus is raised, the air thus imprisoned expands, and
prevents the entry of more water from nearer the surface.

[Illustration: FIG. 205.--Ice-box for transmission of water samples,
etc.]

_Transport of Sample._--If the examination of the sample cannot be
commenced immediately, steps must be taken to prevent the multiplication
of the bacteria contained in the water during the interval occupied in
transit from the place of collection to the laboratory. To this end an
ice-box such as that shown (in Fig. 205) is essential. It consists of a
double-walled metal cylinder into which slides a cylindrical chamber of
sufficient capacity to accommodate four of the 60 c.c. bottles; this in
turn is covered by a metal disc--the three portions being bolted
together by thumb screws through the overhanging flanges. When in use,
place the bottles, rolled in cotton-wool, in the central chamber, pack
the space between the walls with pounded ice, securely close the metal
box by screwing down the fly nuts, and place it in a felt-lined wooden
case. (It has been shown that whilst bacteria will survive exposure to
the temperature of melting ice, practically none will multiply at this
temperature.)

On reaching the laboratory, the method of examination consists in adding
measured quantities of the water sample to several tubes of nutrient
media previously liquefied by heat, pouring plate cultivations from each
of these tubes, incubating at a suitable temperature, and finally
counting the colonies which make their appearance on the plates.

_Apparatus Required_:

Plate-levelling stand.
Case of sterile plates.
Case of sterile pipettes, 1 c.c. (in tenths of a cubic centimetre).
Case of sterile pipettes, 10 c.c. (in tenths of a cubic centimetre).
Case of sterile capsules, 25 c.c. capacity.
Tubes of nutrient gelatine.
Tubes of nutrient agar.
Tubes of wort gelatine.
One 250 c.c. flask of sterile distilled water.
Ta